Optical spectrometers are devices that separate an optical signal into a plurality of component optical signals based on a wavelength of each component. Thus, a spectrometric device allows for accurate measurement of light within separate wavelength ranges or channels. The measurement of this light allows for determinations to be made about the light being analysed.
It is well known to photograph a spectrum produced by passing light through a gaseous sample in order to classify the sample. In high school chemistry text books, readers are shown pictures of spectra from different chemical samples. Similarly, spectra of burning chemicals are also useful in classifying a chemical composition of the products and reactants.
In more recent times, computers and photodetectors have replaced the old film, camera, and human vision approach to spectroscopy. Currently, a state of the art spectrometer disperses a received light signal onto a surface of a photodetector array. By dispersing the light along a linear path, a linear array is supported. Each detector in the array detects light incident thereon and provides a signal based on the intensity of incident light to a computer system for analysis. The computer then filters the signals to remove noise and processes the spectrum to determine information relating thereto. Spectrometers have gained widespread acceptance in science and are, at present, sufficiently precise and accurate.
Unfortunately, these spectrometers described above are large and costly. As such, they are not well suited to in situ applications or to installation within fibre optic networks.
There are two main approaches to reducing costs and size of spectrometers. In a first approach, fewer photo detectors are employed resulting in lower resolution of the spectrometer. Such an approach with a fixed photodetector array has been avoided by scientists unless the spectral region of interest is very small because it reduces the resolution—quality—of the resulting information. It has been proposed to move the photodetectors, a mirror within the optical path of the dispersed light, or the dispersive element itself to increase the resolution of the spectrometer. However because these devices have moving parts they are costly, difficult to manufacture, require re-calibration at frequent intervals and, as such, are not preferable for use in optical networks.
In the second approach, smaller optical components are used resulting in insufficient dispersion of light incident on the photo detectors. The resolution of the spectrometer is limited by the spacing of the photo detectors in the array. As such, very small photodetector arrays having small detector elements are necessary. This is problematic since smaller detectors are typically more costly and less sensitive.
In fibre optics, spectrometers are used to monitor channelised optical signals. Such a spectrometer is generally referred to as a monitor. For integrated monitors, if information other than light intensity is required—several photodetectors are located within the dispersed light of a single channel. Thus, a four channel optical monitor requires at least 12 photo detectors. Since current photodetectors are supplied in strips of up to 128 photodetectors, this allows monitoring of just over 40 channels. Of course, smaller photodetector arrays are far less costly and, as such, would be preferred. For this reason, most available monitors operate on 4, 8, or 16 channels. Alternatively, monitors only monitor signal intensity thereby requiring only one detector per channel. It would be advantageous to provide a spectrometer capable of monitoring more information than merely intensity data of light without requiring complex and costly photo detector arrays and the additional equipment required to interpret the output of a complex photodetector array.
Prior art optical spectrometers utilising scanning gratings with moving parts allow for an increase in the information measurable by the device. There is, however, a need for an optical spectrometer having no moving parts and a capacity for broadband operation with selectable information measurable for light within channels within an input optical signal.